In the field of digital data network communications, data sent by a source to a destination is generally divided into unit aggregations that are switched across a network. These unit aggregations of data are referred to in the art variously by the terms cells, datagrams, frames, and packets. A Wide Area Network ("WAN") is generally an interconnection of two types of components: transmission lines (also called circuits, channels, or trunks), and switches that connect two or more transmission lines. Cells are switched from transmission line to transmission line by switches. Each switch transmits a cell via a transmission line to another switch or to its destination.
Data from different sources have different data transfer requirements, and exhibit different traffic patterns across a network. For example, computer data often exhibits an unpredictable and variable traffic pattern. Such data are often transferred from source to destination in bursts, so that there are some time periods when a relatively large volume of data is transferred across a network and other time periods when a relatively small volume of data is transferred. In contrast, real time voice and uncompressed video data usually have a constant transfer rate and usually require that data cells be delivered with appropriately small transfer delay variation. Real time compressed video transfers in bursts and is unpredictable, and also requires small transfer delay variation.
Service providers traditionally have provided circuit switched services across a Wide Area Network ("WAN") in which a dial up or leased circuit is effectively rented in its entirety by a service customer for the duration of a call. For example, a T1 circuit provides the capability of a continuous data transfer rate of 1.536M bits of information per second. Such a circuit switched T1 connection provides the entire 1.536M bits per second capability from source to destination, regardless of how many bits per second of data are actually being transmitted by the service customer. This circuit switched service effectively provides the equivalent of a direct wire through the WAN from the termination located at one service customer premise to the termination located at one other service customer premise.
Such a service customer usually contracts for this circuit switched service based on the maximum data transfer (or bandwidth) requirements of the customer by arranging for a circuit capable of providing those bandwith requirements. Often this dedication of circuit capability results in the waste of bandwidth and network resources, because the service customer does not always require the maximum data transfer rate. A circuit switched connection cannot accommodate a burst increase in data transfer rate that is above the maximum capacity of the circuit. If a service customer claims that adequate circuit switched service is not being provided, the circuit switched service provider typically runs tests to determine whether the line is operating properly.
One type of test operates on an in-service communications link meaning that the customer line remains in operation while the test is being executed and the test does not interfere with the customer use of the communications link. The test is transparent to the customer. One example of a test performed on an in-service circuit switched communications link is a test that uses management overhead information multiplexed into the T1 circuit framing. This test can verify correct operation of the circuit switched connection without interfering with the in-service communications link. Another type of test is an out of service test, for example a Bit Error Rate ("BER") test. A BER test requires the full use of the customer circuit for the test. A BER test therefore cannot operate on an in-service circuit switched communications link.
In recent years, service providers have offered a "virtual circuit" model of service. Virtual circuits can be multiplexed onto traditional T1, T3, OC3, or OC12 type circuits at the point of service customer access. Several virtual circuits may be provided on the same access line that terminate at different geographic locations, so a service customer may reach many endpoints using a single access line. Service customers contract for a specific quality of service to be provided by each virtual circuit. The service provider can take advantage of the statistically variable nature of service customers' bandwidth demands and use the bandwidth available on a line more efficiently, rather than dedicate the full bandwidth to a circuit that may not be fully used for periods of time. The service provider may be able to accommodate service customers burst traffic for short periods of time. The service customer may therefore have his communications needs met at a lower cost.
One example of a technology that provides virtual circuits is Asynchronous Transfer Mode ("ATM"). ATM networks have lines and switches like traditional networks, but the switches operate differently. In ATM, all information is transmitted in small fixed-size cells. ATM cells are 53 bytes long; the first five bytes contain header information and the other 48 bytes contain message payload. The cells are switched individually across the network. Because the cells are relatively small they can be switched at very high speeds. Such cell switching can handle both constant rate traffic (audio, video) and variable rate traffic (data). Cell switching can also provide the ability to broadcast. Although ATM networks switch each cell individually, ATM is connection-oriented. Data transfer requires that first a message be sent to set up the connection. Subsequent cells all follow the same path to the destination. This connection is a virtual circuit.
Asynchronous transfer mode virtual circuits are typically purchased with a specified set of quality of service ("QoS") parameters. The ATM standards, for example International Telecommunications Union standard ITU-T I.356, define a number of these QoS parameters. For each parameter, a worst case value is specified, and the service provider is required to meet or exceed the worst case value. In some cases the parameter is a minimum and in others it is a maximum. The Peak Cell Rate ("PCR") is the maximum rate at which the sender is allowed to send cells. The Sustained Cell Rate ("SCR") is the expected or required cell transfer rate averaged over a longer time interval. The Cell Delay Variation ("CDV") specifies how much variation will be present in cell arrival times relative to cell transmission times, in other words, how uniformly the cells are delivered. The Cell Transfer Delay ("CTD") is the average transit time from source to destination. The Cell Error Ratio ("CER") is the fraction of cells that are delivered on time but with one or more wrong bits. The Cell Loss Ratio ("CLR") is the fraction of transmitted cells that are not delivered within a maximum allowable cell transfer delay. The Severely-Errored Cell Block Ratio ("SECBR") is the number of blocks of a given length that contain n cells with one or more wrong bits, where n is typically much greater than one. The Cell Misinsertion Rate ("CMR") is the number of cells/second that are delivered to the virtual circuit destination but were not sent by the source but rather were inadvertently created by the network.
Another example of a technology that provides virtual circuits is Frame Relay. Frame relay provides a very simple protocol for sending data from one point to another. The service customer can send cells of up to approximately 1600 bytes between two virtually connected points. Like ATM, contracting between the service provider and the service customer determines QoS requirements. Measurement of the QoS parameters described above is therefore also useful for frame relay.
Virtual circuit services present a new problem to the service provider. A service provider must be able to test the QoS of a virtual circuit to verify that the virtual circuit meets the contracted parameters. International Telecommunications Union (ITU) standards describe methods to test QoS by sending test streams of cells over the customer virtual circuit. Such tests require that the service customer virtual circuit be turned off, and are therefore an out of service test. The inability to use the customer virtual circuit may be irritating to the service customer. Other tests run simultaneously through the network along with the service customer's virtual circuit, but are separately identified on a test virtual circuit which uses the same path and equal bandwidth. This approach risks stressing the customer virtual circuit such that the test may induce a QoS problem where none actually existed when the test was not running.
U.S. Pat. No. 5,369,634 to Denissen discloses a quality assessment arrangement useful for cells having a header that "contains information identifying the communication to which the cell belongs and a tag" (col. 3 lines 14-16). The Denissen apparatus counts the customer data cells and injects test packets into the customer's cell stream every M cells. "[C]ounter 3 provides the cell count M+1 to the generator 4 and as a consequence the latter then generates a test cell . . . " (col. 3 lines 43-45). The test cell header has "an identification tag indicative of a test cell." The test cell header is therefore different than the customer data cell headers. Denissen has the disadvantage of not being useful for technologies in which a cell header does not have tag information. Without the distinguishing tag information in the header, the Denissen arrangement cannot distinguish between test packets and customer service packets. Also, Denissen sends test packets every M cells, which risks stressing the virtual circuit such that the test may induce a QoS problem where none actually existed when the test was not running.
What is desired is virtual circuit QoS measurement for an in-service virtual circuit in a manner that is accurate and transparent to the user. The present invention permits such measurement.